This invention relates to a thin film of manganese-aluminum alloy and more particularly to a film suitable for generating spontaneous magnetization and magnetic anisotropy as high as those of a bulk of this alloy.
This invention is further directed to a magnetic recording medium, such as a magnetic disc, etc.
The present invention relates to a structure of ferromagnetic film having a high saturation magnetization necessary for drastic improvement of characteristics of magnetic devices such as magnetic heads, etc. in magnetic recording apparatuses and to a method for producing the same.
In magnetic recording media such as magnetic discs, etc., an increase in recording density is the most important task, and thus it is important in any of systems of in-plane magnetic recording and perpendicular magnetic recording to optimize the characteristics of magnetic film. Particularly important characteristics include a saturation magnetic flux density, a ratio of residual magnetization to saturation magnetization, i.e., so-called squareness ratio, and a coercivity.
These magnetic characteristics have so far been controlled mainly by adjusting the film composition, that is, by changing the composition ratio of constituent elements in the film or adding a very small amount of other elements to the film. For example, in case of Co-Cr-Pt alloy film disclosed in Japanese Patent Application Kokai (Laid-Open) No. Hei 1-28383, the magnetic characteristics are controlled by adjusting the composition ratio of Co, Cr, and Pt, film thickness, thickness of Cr layer as an underlayer, etc.
Development of magnetic members having a high saturation magnetization which cannot be obtained by conventional magnetic films has been desired for further improvement of magnetic devices. Alloys composed mainly of iron have been studied as magnetic members having a high saturation magnetization. However, it has been found that the alloys have critical saturation magnetization values which are almost equal to the saturation magnetization value of iron (2.2 tesla).
Recently, multi-layer films comprising different kinds of thin layers of different elements as alternatingly laid upon one another have attracted attention to attain the aim to make the alloy saturation magnetization surpass the critical value. As one of the multi-layer films, a multi-layer film comprising manganese and aluminum is disclosed in Japanese Patent Application No. Hei 1-229151. The disclosed multi-layer film shows good magnetic characteristics, particularly a high saturation magnetization, but has suffered from the problem that the film is apt to undergo changes in the characteristics by the heat treatment at about 300.degree. C. which is conducted in the processing of the film.
It is known that an alloy of manganese (Mn) and aluminum (Al) becomes ferromagnetic in some crystal structures and its bulk crystal is applied to a permanent magnet, etc. Recently, attempts have been made to make a thin film from this material. One example is a film of Mn-Al formed by sputtering, as disclosed in J. Appli. Phys. 61 (1987) pp 4281-4283, where an atomic ratio of Mn to Al is constant throughout the film.
The foregoing prior art has such a problem that, even if the film-forming conditions are changed, the resulting films have a maximum spontaneous magnetization of 120 emu/cc, which is about one-fourth of that of the bulk of the alloy, because the resulting films contain a non-magnetic structure (.epsilon. phase) together with a structure generating a ferromagnetic property (.tau. phase), even if the atomic ratio of Mn to Al is constant.